Page:Handbook of Ophthalmology (3rd edition).djvu/36

30 nution in the acuteness of vision, and it is to be expressed in the manner above described.

It is in many respects more convenient to allow the distance between the test letters and the patient's eye to remain unchanged, and to write as numerator of the fraction the number which is seen with normal vision at that distance, and as denominator the number which the patient actually sees; for instance, if not No. XX but No. LXX are the smallest letters to be seen at a distance of twenty feet, the acuteness of vision is j^.

In using these test letters it should be remembered that it is by average daylight that the letters are recognized at the given distances. AVith intense illumination they can be seen at even a greater distance. The changes in the intensity of daylight, which are dependent upon variations in the weather, are a source of error difficult to avoid.

The proposition to abandon daylight and conduct this examination by artificial light is also objectionable, for artificial light is likewise subject to considerable variations of intensity, is difficult to control, and further, when we conduct a patient out of the daylight into a room artificially lighted in order there to try his acuteness of vision, an adaptation of the retina to the changed illumination must first take place. The laws according to which the retina adapts itself, especially in pathological conditions, to light of different degrees of intensity, are as yet imperfectly understood.

The unavoidable variations in the intensity of daylight seem the simplest to deal with. In my own private practice I have found it convenient to use a modification of the usual expression for the acuteness of vision. If, according to Snellen's plan, the denominator of the fraction be the number of the test type recognized by the patient, and the numerator the number of feet at which he sees them, then it is only the denominator which is affected by changes in the illumination; the errors from this source become less when we subject the numerator also to the same influences. I prefer, therefore, to write as numerator of the fraction that number of the test type which I can recognize simultaneously with and at the same distance as the patient; as denominator, I write the number of the type which the patient reads. In this way some but not all sources of error are avoided.